Neurostimulation in a neural fulcrum zone for the treatment of chronic cardiac dysfunction

ABSTRACT

Systems and methods are provided for delivering neurostimulation therapies to patients for treating chronic heart failure. A neural fulcrum zone is identified and ongoing neurostimulation therapy is delivered within the neural fulcrum zone. This neural fulcrum zone corresponds to a combination of stimulation parameters at which autonomic engagement is achieved, while the tachycardia-inducing stimulation effects are offset by the bradycardia-inducing effects, thereby minimizing side effects such as significant heart rate changes while providing a therapeutic level of stimulation.

FIELD

This application relates to neuromodulation.

BACKGROUND

Chronic heart failure (CHF) and other forms of chronic cardiac dysfunction (CCD) may be related to an autonomic imbalance of the sympathetic and parasympathetic nervous systems that, if left untreated, can lead to cardiac arrhythmogenesis, progressively worsening cardiac function and eventual patient death. CHF is pathologically characterized by an elevated neuroexitatory state and is accompanied by physiological indications of impaired arterial and cardiopulmonary baroreflex function with reduced vagal activity.

CHF triggers compensatory activations of the sympathoadrenal (sympathetic) nervous system and the renin-angiotensin-aldosterone hormonal system, which initially helps to compensate for deteriorating heart-pumping function, yet, over time, can promote progressive left ventricular dysfunction and deleterious cardiac remodeling. Patients suffering from CHF are at increased risk of tachyarrhythmias, such as atrial fibrillation (AF), ventricular tachyarrhythmias (ventricular tachycardia (VT) and ventricular fibrillation (VF)), and atrial flutter, particularly when the underlying morbidity is a form of coronary artery disease, cardiomyopathy, mitral valve prolapse, or other valvular heart disease. Sympathoadrenal activation also significantly increases the risk and severity of tachyarrhythmias due to neuronal action of the sympathetic nerve fibers in, on, or around the heart and through the release of epinephrine (adrenaline), which can exacerbate an already-elevated heart rate.

The standard of care for managing CCD in general continues to evolve. For instance, new therapeutic approaches that employ electrical stimulation of neural structures that directly address the underlying cardiac autonomic nervous system imbalance and dysregulation have been proposed. In one form, controlled stimulation of the cervical vagus nerve beneficially modulates cardiovascular regulatory function. Vagus nerve stimulation (VNS) has been used for the clinical treatment of drug-refractory epilepsy and depression, and more recently has been proposed as a therapeutic treatment of heart conditions such as CHF. For instance, VNS has been demonstrated in canine studies as efficacious in simulated treatment of AF and heart failure, such as described in Zhang et al., “Chronic Vagus Nerve Stimulation Improves Autonomic Control and Attenuates Systemic Inflammation and Heart Failure Progression in a Canine High-Rate Pacing Model,” Circ Heart Fail 2009, 2, pp. 692-699 (Sep. 22, 2009), the disclosure of which is incorporated by reference. The results of a multi-center open-label phase II study in which chronic VNS was utilized for CHF patients with severe systolic dysfunction is described in De Ferrari et al., “Chronic Vagus Nerve Stimulation: A New and Promising Therapeutic Approach for Chronic Heart Failure,” European Heart Journal, 32, pp. 847-855 (Oct. 28, 2010).

VNS therapy commonly requires implantation of a neurostimulator, a surgical procedure requiring several weeks of recovery before the neurostimulator can be activated and a patient can start receiving VNS therapy. Even after the recovery and activation of the neurostimulator, a full therapeutic dose of VNS is not immediately delivered to the patient to avoid causing significant patient discomfort and other undesirable side effects. Instead, to allow the patient to adjust to the VNS therapy, a titration process is utilized in which the intensity is gradually increased over a period of time under a control of a physician, with the patient given time between successive increases in VNS therapy intensity to adapt to the new intensity. As stimulation is chronically applied at each new intensity level, the patient's side effect threshold gradually increases, allowing for an increase in intensity during subsequent titration sessions.

Conventional general therapeutic alteration of cardiac vagal efferent activation through electrical stimulation targets only the efferent nerves of the parasympathetic nervous system, such as described in Sabbah et al., “Vagus Nerve Stimulation in Experimental Heart Failure,” Heart Fail. Rev., 16:171-178 (2011), the disclosure of which is incorporated by reference. The Sabbah paper discusses canine studies using a vagus nerve stimulation system, manufactured by BioControl Medical Ltd., Yehud, Israel, which includes an electrical pulse generator, right ventricular endocardial sensing lead, and right vagus nerve cuff stimulation lead. The sensing lead enables stimulation of the right vagus nerve in a highly specific manner, which includes closed-loop synchronization of the vagus nerve stimulation pulse to the cardiac cycle. An asymmetric tri-polar nerve cuff electrode is implanted on the right vagus nerve at the mid-cervical position. The electrode provides cathodic induction of action potentials while simultaneously applying asymmetric anodal block that lead to preferential activation of vagal efferent fibers. Electrical stimulation of the right cervical vagus nerve is delivered only when heart rate is above a preset threshold. Stimulation is provided at an intensity intended to reduce basal heart rate by ten percent by preferential stimulation of efferent vagus nerve fibers leading to the heart while blocking afferent neural impulses to the brain. Although effective in partially restoring baroreflex sensitivity, increasing left ventricular ejection fraction, and decreasing left ventricular end diastolic and end systolic volumes, a portion of the therapeutic benefit is due to incidental recruitment of afferent parasympathetic nerve fibers in the vagus. Efferent stimulation alone is less effective than bidirectional stimulation at restoring autonomic balance.

Accordingly, a need remains for an approach to efficiently providing neurostimulation therapy, and, in particular, to neurostimulation therapy for treating chronic cardiac dysfunction and other conditions.

SUMMARY

In accordance with embodiments of the present invention, a neural fulcrum zone is identified and ongoing neurostimulation therapy is delivered within the neural fulcrum zone. This neural fulcrum zone corresponds to a combination of stimulation parameters at which autonomic engagement is achieved, while the tachycardia-inducing stimulation effects are offset by the bradycardia-inducing effects, thereby minimizing side effects such as significant heart rate changes while providing a therapeutic level of stimulation.

In accordance with embodiments of the present invention, a method of operating an implantable medical device (IMD) comprising neurostimulator coupled to an electrode assembly is provided. The method includes: activating the IMD to deliver to the patient a plurality of stimulation signals at a first frequency, each stimulation signal having at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals; monitoring a heart rate of the patient during said delivery of the plurality of stimulation signals, wherein tachycardia is detected during a first subset of the plurality of stimulation signals and bradycardia is detected during a second subset of the plurality of stimulation signals; and identifying a neural fulcrum zone corresponding to a third subset of the plurality of stimulation signals during which a transition heart rate response is detected, said transition heart rate response being between tachycardia and bradycardia.

In accordance with embodiments of the present invention, a method of identifying a set of parameter settings for an implantable medical device (IMD) for treating chronic cardiac dysfunction is provided, including: activating the IMD to deliver to the patient a plurality of stimulation signals at a first frequency, each stimulation signal having at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals; monitoring a heart rate of the patient during said delivery of the plurality of stimulation signals, wherein tachycardia is detected during a first subset of the plurality of stimulation signals and bradycardia is detected during a second subset of the plurality of stimulation signals; and identifying a neural fulcrum zone corresponding to a third subset of the plurality of stimulation signals during which a transition heart rate response is detected, said transition heart rate response being between the tachycardia and the bradycardia.

In accordance with embodiments of the present invention, a neurostimulation system is provided, including: an implantable medical device (IMD) comprising neurostimulator coupled to an electrode assembly, said neurostimulator adapted to deliver a stimulation signal to a patient; and a control system. The control system is programmed to: monitor a heart rate of a patient implanted with the IMD during delivery to the patient a plurality of stimulation signals at a first frequency, each stimulation signal having at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals; identify a first subset of the plurality of stimulation signals during which tachycardia is detected; identify a second subset of the plurality of stimulation signals during which bradycardia is detected; and identify a neural fulcrum zone corresponding to a third subset of the plurality of stimulation signals during which a transition heart rate response is detected, said transition heart rate response between tachycardia and bradycardia.

Still other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein are described embodiments by way of illustrating the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, all without departing from the spirit and the scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front anatomical diagram showing, by way of example, placement of an implantable vagus stimulation device in a male patient, in accordance with one embodiment.

FIGS. 2A and 2B are diagrams respectively showing the implantable neurostimulator and the simulation therapy lead of FIG. 1.

FIG. 3 is a diagram showing an external programmer for use with the implantable neurostimulator of FIG. 1.

FIG. 4 is a diagram showing electrodes provided as on the stimulation therapy lead of FIG. 2 in place on a vagus nerve in situ.

FIG. 5 is a graph showing, by way of example, the relationship between the targeted therapeutic efficacy and the extent of potential side effects resulting from use of the implantable neurostimulator of FIG. 1.

FIG. 6 is a graph showing, by way of example, the optimal duty cycle range based on the intersection depicted in FIG. 3.

FIG. 7 is a timing diagram showing, by way of example, a stimulation cycle and an inhibition cycle of VNS as provided by implantable neurostimulator of FIG. 1.

FIGS. 8A-8C are illustrative charts reflecting a heart rate response to gradually increased stimulation intensity at different frequencies.

FIG. 9 illustrates a method of operating an implantable medical device comprising neurostimulator coupled to an electrode assembly.

DETAILED DESCRIPTION

CHF and other cardiovascular diseases cause derangement of autonomic control of the cardiovascular system, favoring increased sympathetic and decreased parasympathetic central outflow. These changes are accompanied by elevation of basal heart rate arising from chronic sympathetic hyperactivation along the neurocardiac axis.

The vagus nerve is a diverse nerve trunk that contains both sympathetic and parasympathetic fibers, and both afferent and efferent fibers. These fibers have different diameters and myelination, and subsequently have different activation thresholds. This results in a graded response as intensity is increased. Low intensity stimulation results in a progressively greater tachycardia, which then diminishes and is replaced with a progressively greater bradycardia response as intensity is further increased. Peripheral neurostimulation therapies that target the fluctuations of the autonomic nervous system have been shown to improve clinical outcomes in some patients. Specifically, autonomic regulation therapy results in simultaneous creation and propagation of efferent and afferent action potentials within nerve fibers comprising the cervical vagus nerve. The therapy directly improves autonomic balance by engaging both medullary and cardiovascular reflex control components of the autonomic nervous system. Upon stimulation of the cervical vagus nerve, action potentials propagate away from the stimulation site in two directions, efferently toward the heart and afferently toward the brain. Efferent action potentials influence the intrinsic cardiac nervous system and the heart and other organ systems, while afferent action potentials influence central elements of the nervous system.

An implantable vagus nerve stimulator, such as used to treat drug-refractory epilepsy and depression, can be adapted for use in managing chronic cardiac dysfunction (CCD) through therapeutic bi-directional vagus nerve stimulation. FIG. 1 is a front anatomical diagram showing, by way of example, placement of an implantable medical device (e.g., a vagus nerve stimulation (VNS) system 11, as shown in FIG. 1) in a male patient 10, in accordance with embodiments of the present invention. The VNS provided through the stimulation system 11 operates under several mechanisms of action. These mechanisms include increasing parasympathetic outflow and inhibiting sympathetic effects by inhibiting norepinephrine release and adrenergic receptor activation. More importantly, VNS triggers the release of the endogenous neurotransmitter acetylcholine and other peptidergic substances into the synaptic cleft, which has several beneficial anti-arrhythmic, anti-apoptotic, and anti-inflammatory effects as well as beneficial effects at the level of the central nervous system.

The implantable vagus stimulation system 11 comprises an implantable neurostimulator or pulse generator 12 and a stimulating nerve electrode assembly 125. The stimulating nerve electrode assembly 125, preferably comprising at least an electrode pair, is conductively connected to the distal end of an insulated, electrically conductive lead assembly 13 and electrodes 14. The electrodes 14 may be provided in a variety of forms, such as, e.g., helical electrodes, probe electrodes, cuff electrodes, as well as other types of electrodes. The implantable vagus stimulation system 11 can be remotely accessed following implant through an external programmer, such as the programmer 40 shown in FIG. 3 and described in further detail below. The programmer 40 can be used by healthcare professionals to check and program the neurostimulator 12 after implantation in the patient 10. In some embodiments, an external magnet may provide basic controls, such as described in commonly assigned U.S. Pat. No. 8,600,505, entitled “Implantable Device For Facilitating Control Of Electrical Stimulation Of Cervical Vagus Nerves For Treatment Of Chronic Cardiac Dysfunction,” the disclosure of which is incorporated by reference. For further example, an electromagnetic controller may enable the patient 10 or healthcare professional to interact with the implanted neurostimulator 12 to exercise increased control over therapy delivery and suspension, such as described in commonly-assigned U.S. Pat. No. 8,571,654, entitled “Vagus Nerve Neurostimulator With Multiple Patient-Selectable Modes For Treating Chronic Cardiac Dysfunction,” the disclosure of which is incorporated by reference. For further example, an external programmer may communicate with the neurostimulation system 11 via other wired or wireless communication methods, such as, e.g., wireless RF transmission. Together, the implantable vagus stimulation system 11 and one or more of the external components form a VNS therapeutic delivery system.

The neurostimulator 12 is typically implanted in the patient's right or left pectoral region generally on the same side (ipsilateral) as the vagus nerve 15, 16 to be stimulated, although other neurostimulator-vagus nerve configurations, including contra-lateral and bi-lateral are possible. A vagus nerve typically comprises two branches that extend from the brain stem respectively down the left side and right side of the patient, as seen in FIG. 1. The electrodes 14 are generally implanted on the vagus nerve 15, 16 about halfway between the clavicle 19 a-b and the mastoid process. The electrodes may be implanted on either the left or right side. The lead assembly 13 and electrodes 14 are implanted by first exposing the carotid sheath and chosen branch of the vagus nerve 15, 16 through a latero-cervical incision (perpendicular to the long axis of the spine) on the ipsilateral side of the patient's neck 18. The helical electrodes 14 are then placed onto the exposed nerve sheath and tethered. A subcutaneous tunnel is formed between the respective implantation sites of the neurostimulator 12 and helical electrodes 14, through which the lead assembly 13 is guided to the neurostimulator 12 and securely connected.

In one embodiment, the neural stimulation is provided as a low level maintenance dose independent of cardiac cycle. The stimulation system 11 bi-directionally stimulates either the left vagus nerve 15 or the right vagus nerve 16. However, it is contemplated that multiple electrodes 14 and multiple leads 13 could be utilized to stimulate simultaneously, alternatively or in other various combinations. Stimulation may be through multimodal application of continuously-cycling, intermittent and periodic electrical stimuli, which are parametrically defined through stored stimulation parameters and timing cycles. Both sympathetic and parasympathetic nerve fibers in the vagosympathetic complex are stimulated. A study of the relationship between cardiac autonomic nerve activity and blood pressure changes in ambulatory dogs is described in J. Hellyer et al., “Autonomic Nerve Activity and Blood Pressure in Ambulatory Dogs,” Heart Rhythm, Vol. 11(2), pp. 307-313 (February 2014). Generally, cervical vagus nerve stimulation results in propagation of action potentials from the site of stimulation in a bi-directional manner. The application of bi-directional propagation in both afferent and efferent directions of action potentials within neuronal fibers comprising the cervical vagus nerve improves cardiac autonomic balance. Afferent action potentials propagate toward the parasympathetic nervous system's origin in the medulla in the nucleus ambiguus, nucleus tractus solitarius, and the dorsal motor nucleus, as well as towards the sympathetic nervous system's origin in the intermediolateral cell column of the spinal cord. Efferent action potentials propagate toward the heart 17 to activate the components of the heart's intrinsic nervous system. Either the left or right vagus nerve 15, 16 can be stimulated by the stimulation system 11. The right vagus nerve 16 has a moderately lower (approximately 30%) stimulation threshold than the left vagus nerve 15 for heart rate effects at the same stimulation frequency and pulse width.

The VNS therapy is delivered autonomously to the patient's vagus nerve 15, 16 through three implanted components that include a neurostimulator 12, lead assembly 13, and electrodes 14. FIGS. 2A and 2B are diagrams respectively showing the implantable neurostimulator 12 and the stimulation lead assembly 13 of FIG. 1. In one embodiment, the neurostimulator 12 can be adapted from a VNS Therapy DemiPulse Model 103 or AspireSR Model 106 pulse generator, manufactured and sold by Cyberonics, Inc., Houston, Tex., although other manufactures and types of implantable VNS neurostimulators could also be used. The stimulation lead assembly 13 and electrodes 14 are generally fabricated as a combined assembly and can be adapted from a Model 302 lead, PerenniaDURA Model 303 lead, or PerenniaFLEX Model 304 lead, also manufactured and sold by Cyberonics, Inc., in two sizes based, for example, on a helical electrode inner diameter, although other manufactures and types of single-pin receptacle-compatible therapy leads and electrodes could also be used.

Referring first to FIG. 2A, the system 20 may be configured to provide multimodal vagus nerve stimulation. In a maintenance mode, the neurostimulator 12 is parametrically programmed to deliver continuously-cycling, intermittent and periodic ON-OFF cycles of VNS. Such delivery produces action potentials in the underlying nerves that propagate bi-directionally.

The neurostimulator 12 includes an electrical pulse generator that is tuned to improve autonomic regulatory function by triggering action potentials that propagate both afferently and efferently within the vagus nerve 15, 16. The neurostimulator 12 is enclosed in a hermetically sealed housing 21 constructed of a biocompatible, implantation-safe material, such as titanium. The housing 21 contains electronic circuitry 22 powered by a battery 23, such as a lithium carbon monofluoride primary battery or a rechargeable secondary cell battery. The electronic circuitry 22 may be implemented using complementary metal oxide semiconductor integrated circuits that include a microprocessor controller that executes a control program according to stored stimulation parameters and timing cycles; a voltage regulator that regulates system power; logic and control circuitry, including a recordable memory 29 within which the stimulation parameters are stored, that controls overall pulse generator function, receives and implements programming commands from the external programmer, or other external source, collects and stores telemetry information, processes sensory input, and controls scheduled and sensory-based therapy outputs; a transceiver that remotely communicates with the external programmer using radio frequency signals; an antenna, which receives programming instructions and transmits the telemetry information to the external programmer; and a reed switch 30 that provides remote access to the operation of the neurostimulator 12 using an external programmer, a simple patient magnet, or an electromagnetic controller. The recordable memory 29 can include both volatile (dynamic) and persistent (static) forms of memory, such as firmware within which the stimulation parameters and timing cycles can be stored. Other electronic circuitry and components are possible.

The neurostimulator 12 includes a header 24 to securely receive and connect to the lead assembly 13. In one embodiment, the header 24 encloses a receptacle 25 into which a single pin for the lead assembly 13 can be received, although two or more receptacles could also be provided, along with the corresponding electronic circuitry 22. The header 24 internally includes a lead connector block (not shown) and a set of screws 26.

In some embodiments, the housing 21 may also contain a heart rate sensor 31 that is electrically interfaced with the logic and control circuitry, which receives the patient's sensed heart rate as sensory inputs. The heart rate sensor 31 monitors heart rate using an ECG-type electrode. Through the electrode, the patient's heart beat can be sensed by detecting ventricular depolarization. In a further embodiment, a plurality of electrodes can be used to sense voltage differentials between electrode pairs, which can undergo signal processing for cardiac physiological measures, for instance, detection of the P-wave, QRS complex, and T-wave. The heart rate sensor 31 provides the sensed heart rate to the control and logic circuitry as sensory inputs that can be used to determine the onset or presence of arrhythmias, particularly VT, and/or to monitor and record changes in the patient's heart rate over time or in response to applied stimulation signals.

Referring next to FIG. 2B, the lead assembly 13 delivers an electrical signal from the neurostimulator 12 to the vagus nerve 15, 16 via the electrodes 14. On a proximal end, the lead assembly 13 has a lead connector 27 that transitions an insulated electrical lead body to a metal connector pin 28. During implantation, the connector pin 28 is guided through the receptacle 25 into the header 24 and securely fastened in place using the set screws 26 to electrically couple the lead assembly 13 to the neurostimulator 12. On a distal end, the lead assembly 13 terminates with the electrode 14, which bifurcates into a pair of anodic and cathodic electrodes 62 (as further described infra with reference to FIG. 4). In one embodiment, the lead connector 27 is manufactured using silicone and the connector pin 28 is made of stainless steel, although other suitable materials could be used, as well. The insulated lead body 13 utilizes a silicone-insulated alloy conductor material.

In some embodiments, the electrodes 14 are helical and placed around the cervical vagus nerve 15, 16 at the location below where the superior and inferior cardiac branches separate from the cervical vagus nerve. In alternative embodiments, the helical electrodes may be placed at a location above where one or both of the superior and inferior cardiac branches separate from the cervical vagus nerve. In one embodiment, the helical electrodes 14 are positioned around the patient's vagus nerve oriented with the end of the helical electrodes 14 facing the patient's head. In an alternate embodiment, the helical electrodes 14 are positioned around the patient's vagus nerve 15, 16 oriented with the end of the helical electrodes 14 facing the patient's heart 17. At the distal end, the insulated electrical lead body 13 is bifurcated into a pair of lead bodies that are connected to a pair of electrodes. The polarity of the electrodes could be configured into a monopolar cathode, a proximal anode and a distal cathode, or a proximal cathode and a distal anode.

The neurostimulator 12 may be interrogated prior to implantation and throughout the therapeutic period with a healthcare provider-operable control system comprising an external programmer and programming wand (shown in FIG. 3) for checking proper operation, downloading recorded data, diagnosing problems, and programming operational parameters, such as described in commonly-assigned U.S. Pat. Nos. 8,600,505 and 8,571,654, cited supra. FIG. 3 is a diagram showing an external programmer 40 for use with the implantable neurostimulator 12 of FIG. 1. The external programmer 40 includes a healthcare provider operable programming computer 41 and a programming wand 42. Generally, use of the external programmer is restricted to healthcare providers, while more limited manual control is provided to the patient through “magnet mode.”

In one embodiment, the external programmer 40 executes application software 45 specifically designed to interrogate the neurostimulator 12. The programming computer 41 interfaces to the programming wand 42 through a wired or wireless data connection. The programming wand 42 can be adapted from a Model 201 Programming Wand, manufactured and sold by Cyberonics, Inc., and the application software 45 can be adapted from the Model 250 Programming Software suite, licensed by Cyberonics, Inc. Other configurations and combinations of external programmer 40, programming wand 42 and application software 45 are possible.

The programming computer 41 can be implemented using a general purpose programmable computer and can be a personal computer, laptop computer, ultrabook computer, netbook computer, handheld computer, tablet computer, smart phone, or other form of computational device. In one embodiment, the programming computer is a tablet computer that may operate under the iOS operating system from Apple Inc., such as the iPad from Apple Inc., or may operate under the Android operating system from Google Inc., such as the Galaxy Tab from Samsung Electronics Co., Ltd. In an alternative embodiment, the programming computer is a personal digital assistant handheld computer operating under the Pocket-PC, Windows Mobile, Windows Phone, Windows RT, or Windows operating systems, licensed by Microsoft Corporation, Redmond, Wash., such as the Surface from Microsoft Corporation, the Dell Axim X5 and X50 personal data assistants, sold by Dell, Inc., Round Top, Tex., the HP Jornada personal data assistant, sold by Hewlett-Packard Company, Palo Alto, Tex. The programming computer 41 functions through those components conventionally found in such devices, including, for instance, a central processing unit, volatile and persistent memory, touch-sensitive display, control buttons, peripheral input and output ports, and network interface. The computer 41 operates under the control of the application software 45, which is executed as program code as a series of process or method modules or steps by the programmed computer hardware. Other assemblages or configurations of computer hardware, firmware, and software are possible.

Operationally, the programming computer 41, when connected to a neurostimulator 12 through wireless telemetry using the programming wand 42, can be used by a healthcare provider to remotely interrogate the neurostimulator 12 and modify stored stimulation parameters. The programming wand 42 provides data conversion between the digital data accepted by and output from the programming computer and the radio frequency signal format that is required for communication with the neurostimulator 12. The programming computer 41 may further be configured to receive inputs, such as physiological signals received from patient sensors (e.g., implanted or external). These sensors may be configured to monitor one or more physiological signals, e.g., vital signs, such as body temperature, pulse rate, respiration rate, blood pressure, etc. These sensors may be coupled directly to the programming computer 41 or may be coupled to another instrument or computing device which receives the sensor input and transmits the input to the programming computer 41. The programming computer 41 may monitor, record, and/or respond to the physiological signals in order to effectuate stimulation delivery in accordance with embodiments of the present invention.

The healthcare provider operates the programming computer 41 through a user interface that includes a set of input controls 43 and a visual display 44, which could be touch-sensitive, upon which to monitor progress, view downloaded telemetry and recorded physiology, and review and modify programmable stimulation parameters. The telemetry can include reports on device history that provide patient identifier, implant date, model number, serial number, magnet activations, total ON time, total operating time, manufacturing date, and device settings and stimulation statistics and on device diagnostics that include patient identifier, model identifier, serial number, firmware build number, implant date, communication status, output current status, measured current delivered, lead impedance, and battery status. Other kinds of telemetry or telemetry reports are possible.

During interrogation, the programming wand 42 is held by its handle 46 and the bottom surface 47 of the programming wand 42 is placed on the patient's chest over the location of the implanted neurostimulator 12. A set of indicator lights 49 can assist with proper positioning of the wand and a set of input controls 48 enable the programming wand 42 to be operated directly, rather than requiring the healthcare provider to awkwardly coordinate physical wand manipulation with control inputs via the programming computer 41. The sending of programming instructions and receipt of telemetry information occur wirelessly through radio frequency signal interfacing. Other programming computer and programming wand operations are possible.

Preferably, the electrodes 14 are helical and placed on the cervical vagus nerve 15, 16 at the location below where the superior and inferior cardiac branches separate from the cervical vagus nerve. FIG. 4 is a diagram showing the helical electrodes 14 provided as on the stimulation lead assembly 13 of FIG. 2 in place on a vagus nerve 15, 16 in situ 50. Although described with reference to a specific manner and orientation of implantation, the specific surgical approach and implantation site selection particulars may vary, depending upon physician discretion and patient physical structure.

Under one embodiment, helical electrodes 14 may be positioned on the patient's vagus nerve 61 oriented with the end of the helical electrodes 14 facing the patient's head. At the distal end, the insulated electrical lead body 13 is bifurcated into a pair of lead bodies 57, 58 that are connected to a pair of electrodes 51, 52. The polarity of the electrodes 51, 52 could be configured into a monopolar cathode, a proximal anode and a distal cathode, or a proximal cathode and a distal anode. In addition, an anchor tether 53 is fastened over the lead bodies 57, 58 that maintains the helical electrodes' position on the vagus nerve 61 following implant. In one embodiment, the conductors of the electrodes 51, 52 are manufactured using a platinum and iridium alloy, while the helical materials of the electrodes 51, 52 and the anchor tether 53 are a silicone elastomer.

During surgery, the electrodes 51, 52 and the anchor tether 53 are coiled around the vagus nerve 61 proximal to the patient's head, each with the assistance of a pair of sutures 54, 55, 56, made of polyester or other suitable material, which help the surgeon to spread apart the respective helices. The lead bodies 57, 58 of the electrodes 51, 52 are oriented distal to the patient's head and aligned parallel to each other and to the vagus nerve 61. A strain relief bend 60 can be formed on the distal end with the insulated electrical lead body 13 aligned, for example, parallel to the helical electrodes 14 and attached to the adjacent fascia by a plurality of tie-downs 59 a-b.

The neurostimulator 12 delivers VNS under control of the electronic circuitry 22. The stored stimulation parameters are programmable. Each stimulation parameter can be independently programmed to define the characteristics of the cycles of therapeutic stimulation and inhibition to ensure optimal stimulation for a patient 10. The programmable stimulation parameters include output current, signal frequency, pulse width, signal ON time, signal OFF time, magnet activation (for VNS specifically triggered by “magnet mode”), and reset parameters. Other programmable parameters are possible. In addition, sets or “profiles” of preselected stimulation parameters can be provided to physicians with the external programmer and fine-tuned to a patient's physiological requirements prior to being programmed into the neurostimulator 12, such as described in commonly-assigned U.S. Patent application entitled “Computer-Implemented System and Method for Selecting Therapy Profiles of Electrical Stimulation of Cervical Vagus Nerves for Treatment of Chronic Cardiac Dysfunction,” Ser. No. 13/314,138, filed on Dec. 7, 2011, published as U.S. Patent Publication no. 2013-0158618 A1, pending, the disclosure of which is incorporated by reference.

Therapeutically, the VNS may be delivered as a multimodal set of therapeutic doses, which are system output behaviors that are pre-specified within the neurostimulator 12 through the stored stimulation parameters and timing cycles implemented in firmware and executed by the microprocessor controller. The therapeutic doses include a maintenance dose that includes continuously-cycling, intermittent and periodic cycles of electrical stimulation during periods in which the pulse amplitude is greater than 0 mA (“therapy ON”) and during periods in which the pulse amplitude is 0 mA (“therapy OFF”).

The neurostimulator 12 can operate either with or without an integrated heart rate sensor, such as respectively described in commonly-assigned U.S. Pat. No. 8,577,458, entitled “Implantable Device for Providing Electrical Stimulation of Cervical Vagus Nerves for Treatment of Chronic Cardiac Dysfunction with Leadless Heart Rate Monitoring,” and U.S. patent application, entitled “Implantable Device for Providing Electrical Stimulation of Cervical Vagus Nerves for Treatment of Chronic Cardiac Dysfunction,” Ser. No. 13/314,119, filed on Dec. 7, 2011, pending, the disclosures of which are hereby incorporated by reference herein in their entirety. Additionally, where an integrated leadless heart rate monitor is available, the neurostimulator 12 can provide autonomic cardiovascular drive evaluation and self-controlled titration, such as respectively described in commonly-assigned U.S. patent application entitled “Implantable Device for Evaluating Autonomic Cardiovascular Drive in a Patient Suffering from Chronic Cardiac Dysfunction,” Ser. No. 13/314,133, filed on Dec. 7, 2011, U.S. Patent Publication No. 2013-0158616 A1, pending, and U.S. patent application entitled “Implantable Device for Providing Electrical Stimulation of Cervical Vagus Nerves for Treatment of Chronic Cardiac Dysfunction with Bounded Titration,” Ser. No. 13/314,135, filed on Dec. 7, 2011, U.S. Patent Publication No. 2013-0158617 A1, pending, the disclosures of which are incorporated by reference. Finally, the neurostimulator 12 can be used to counter natural circadian sympathetic surge upon awakening and manage the risk of cardiac arrhythmias during or attendant to sleep, particularly sleep apneic episodes, such as respectively described in commonly-assigned U.S. patent application entitled “Implantable Neurostimulator-Implemented Method For Enhancing Heart Failure Patient Awakening Through Vagus Nerve Stimulation,” Ser. No. 13/673,811, filed on Nov. 9, 2012, pending, the disclosure of which is incorporated by reference.

The VNS stimulation signal may be delivered as a therapy in a maintenance dose having an intensity that is insufficient to elicit undesirable side effects, such as cardiac arrhythmias. The VNS can be delivered with a periodic duty cycle in the range of 2% to 89% with a preferred range of around 4% to 36% that is delivered as a low intensity maintenance dose. Alternatively, the low intensity maintenance dose may comprise a narrow range approximately at 17.5%, such as around 15% to 20%. The selection of duty cycle is a tradeoff among competing medical considerations. The duty cycle is determined by dividing the stimulation ON time by the sum of the ON and OFF times of the neurostimulator 12 during a single ON-OFF cycle. However, the stimulation time may also need to include ramp-up time and ramp-down time, where the stimulation frequency exceeds a minimum threshold (as further described infra with reference to FIG. 7).

FIG. 5 is a graph 70 showing, by way of example, the relationship between the targeted therapeutic efficacy 73 and the extent of potential side effects 74 resulting from use of the implantable neurostimulator 12 of FIG. 1, after the patient has completed the titration process. The graph in FIG. 5 provides an illustration of the failure of increased stimulation intensity to provide additional therapeutic benefit, once the stimulation parameters have reached the neural fulcrum zone, as will be described in greater detail below with respect to FIG. 8. As shown in FIG. 5, the x-axis represents the duty cycle 71. The duty cycle is determined by dividing the stimulation ON time by the sum of the ON and OFF times of the neurostimulator 12 during a single ON-OFF cycle. However, the stimulation time may also include ramp-up time and ramp-down time, where the stimulation frequency exceeds a minimum threshold (as further described infra with reference to FIG. 7). The y-axis represents physiological response 72 to VNS therapy. The physiological response 72 can be expressed quantitatively for a given duty cycle 71 as a function of the targeted therapeutic efficacy 73 and the extent of potential side effects 74, as described infra. The maximum level of physiological response 72 (“max”) signifies the highest point of targeted therapeutic efficacy 73 or potential side effects 74.

Targeted therapeutic efficacy 73 and the extent of potential side effects 74 can be expressed as functions of duty cycle 71 and physiological response 72. The targeted therapeutic efficacy 73 represents the intended effectiveness of VNS in provoking a beneficial physiological response for a given duty cycle and can be quantified by assigning values to the various acute and chronic factors that contribute to the physiological response 72 of the patient 10 due to the delivery of therapeutic VNS. Acute factors that contribute to the targeted therapeutic efficacy 73 include beneficial changes in heart rate variability and increased coronary flow, reduction in cardiac workload through vasodilation, and improvement in left ventricular relaxation. Chronic factors that contribute to the targeted therapeutic efficacy 73 include improved cardiovascular regulatory function, as well as decreased negative cytokine production, increased baroreflex sensitivity, increased respiratory gas exchange efficiency, favorable gene expression, renin-angiotensin-aldosterone system down-regulation, anti-arrhythmic, anti-apoptotic, and ectopy-reducing anti-inflammatory effects. These contributing factors can be combined in any manner to express the relative level of targeted therapeutic efficacy 73, including weighting particular effects more heavily than others or applying statistical or numeric functions based directly on or derived from observed physiological changes. Empirically, targeted therapeutic efficacy 73 steeply increases beginning at around a 5% duty cycle, and levels off in a plateau near the maximum level of physiological response at around a 30% duty cycle. Thereafter, targeted therapeutic efficacy 73 begins decreasing at around a 50% duty cycle and continues in a plateau near a 25% physiological response through the maximum 100% duty cycle.

The intersection 75 of the targeted therapeutic efficacy 73 and the extent of potential side effects 74 represents one optimal duty cycle range for VNS. FIG. 6 is a graph 80 showing, by way of example, the optimal duty cycle range 83 based on the intersection 75 depicted in FIG. 5. The x-axis represents the duty cycle 81 as a percentage of stimulation time over stimulation time plus inhibition time. The y-axis represents therapeutic points 82 reached in operating the neurostimulator 12 at a given duty cycle 81. The optimal duty range 83 is a function 84 of the intersection 75 of the targeted therapeutic efficacy 73 and the extent of potential side effects 74. The therapeutic operating points 82 can be expressed quantitatively for a given duty cycle 81 as a function of the values of the targeted therapeutic efficacy 73 and the extent of potential side effects 74 at their point of intersection in the graph 70 of FIG. 5. The optimal therapeutic operating point 85 (“max”) signifies a tradeoff that occurs at the point of highest targeted therapeutic efficacy 73 in light of lowest potential side effects 74 and that point will typically be found within the range of a 5% to 30% duty cycle 81. Other expressions of duty cycles and related factors are possible.

Therapeutically and in the absence of patient physiology of possible medical concern, such as cardiac arrhythmias, VNS is delivered in a low level maintenance dose that uses alternating cycles of stimuli application (ON) and stimuli inhibition (OFF) that are tuned to activate both afferent and efferent pathways. Stimulation results in parasympathetic activation and sympathetic inhibition, both through centrally-mediated pathways and through efferent activation of preganglionic neurons and local circuit neurons. FIG. 7 is a timing diagram showing, by way of example, a stimulation cycle and an inhibition cycle of VNS 90, as provided by implantable neurostimulator 12 of FIG. 1. The stimulation parameters enable the electrical stimulation pulse output by the neurostimulator 12 to be varied by both amplitude (output current 96) and duration (pulse width 94). The number of output pulses delivered per second determines the signal frequency 93. In one embodiment, a pulse width in the range of 100 to 250 μSec delivers between 0.02 mA and 50 mA of output current at a signal frequency of about 10 Hz, although other therapeutic values could be used as appropriate. In general, the stimulation signal delivered to the patient may be defined by a stimulation parameter set comprising at least an amplitude, a frequency, a pulse width, and a duty cycle.

In one embodiment, the stimulation time is considered the time period during which the neurostimulator 12 is ON and delivering pulses of stimulation, and the OFF time is considered the time period occurring in-between stimulation times during which the neurostimulator 12 is OFF and inhibited from delivering stimulation.

In another embodiment, as shown in FIG. 7, the neurostimulator 12 implements a stimulation time 91 comprising an ON time 92, a ramp-up time 97 and a ramp-down time 98 that respectively precede and follow the ON time 92. Under this embodiment, the ON time 92 is considered to be a time during which the neurostimulator 12 is ON and delivering pulses of stimulation at the full output current 96. Under this embodiment, the OFF time 95 is considered to comprise the ramp-up time 97 and ramp-down time 98, which are used when the stimulation frequency is at least 10 Hz, although other minimum thresholds could be used, and both ramp-up and ramp-down times 97, 98 last two seconds, although other time periods could also be used. The ramp-up time 97 and ramp-down time 98 allow the strength of the output current 96 of each output pulse to be gradually increased and decreased, thereby avoiding deleterious reflex behavior due to sudden delivery or inhibition of stimulation at a programmed intensity.

Therapeutic vagus neural stimulation has been shown to provide cardioprotective effects. Although delivered in a maintenance dose having an intensity that is insufficient to elicit undesirable side effects, such as cardiac arrhythmias, ataxia, coughing, hoarseness, throat irritation, voice alteration, or dyspnea, therapeutic VNS can nevertheless potentially ameliorate pathological tachyarrhythmias in some patients. Although VNS has been shown to decrease defibrillation threshold, VNS has not been shown to terminate VF in the absence of defibrillation. VNS prolongs ventricular action potential duration, so may be effective in terminating VT. In addition, the effect of VNS on the AV node may be beneficial in patients with AF by slowing conduction to the ventricles and controlling ventricular rate.

Neural Fulcrum Zone

As described above, autonomic regulation therapy results in simultaneous creation and propagation of efferent and afferent action potentials within nerve fibers comprising the cervical vagus nerve. Upon stimulation of the cervical vagus nerve, action potentials propagate away from the stimulation site in two directions, efferently toward the heart and afferently toward the brain. Different parameter settings for the neurostimulator 12 may be adjusted to deliver varying stimulation intensities to the patient. The various stimulation parameter settings for current VNS devices include output current amplitude, signal frequency, pulse width, signal ON time, and signal OFF time.

When delivering neurostimulation therapies to patients, it is generally desirable to avoid stimulation intensities that result in either excessive tachycardia or excessive bradycardia. However, researchers have typically utilized the patient's heart rate changes as an indicator of effective recruitment of nerve fibers and engagement of the autonomic nervous system, which is indicative of therapeutic efficacy. Some researchers have proposed that heart rate reduction caused by VNS stimulation is itself beneficial to the patient.

In accordance with embodiments of the present invention, a neural fulcrum zone is identified and neurostimulation therapy is delivered within the neural fulcrum zone. This neural fulcrum zone corresponds to a combination of stimulation parameters at which autonomic engagement is achieved, while the tachycardia-inducing stimulation effects are offset by the bradycardia-inducing effects, thereby minimizing side effects such as significant heart rate changes while providing a therapeutic level of stimulation One method of identifying the neural fulcrum zone is by delivering a plurality of stimulation signals at a fixed frequency but with one or more other parameter settings changed so as to gradually increase the intensity of the stimulation.

FIGS. 8A-8C provide illustrative charts reflecting the location of the neural fulcrum zone. FIG. 8A is a chart 800 illustrating a heart rate response in response to such a gradually increased intensity at a first frequency, in accordance with embodiments of the present invention. In this chart 800, the x-axis represents the intensity level of the stimulation signal, and the y-axis represents the observed heart rate change from the patient's baseline basal heart rate observed when no stimulation is delivered. In this example, the stimulation intensity is increased by increasing the output current amplitude.

A first set 810 of stimulation signals is delivered at a first frequency (e.g., 10 Hz). Initially, as the intensity (e.g., output current amplitude) is increased, a tachycardia zone 851-1 is observed, during which period, the patient experiences a mild tachycardia. As the intensity continues to be increased for subsequent stimulation signals, the patient's heart rate response begins to decrease and eventually enters a bradycardia zone 853-1, in which a bradycardia response is observed in response to the stimulation signals. As described above, the neural fulcrum zone is a range of stimulation parameters at which the functional effects from afferent activation are balanced with the effects from efferent activation to avoid extreme heart rate changes while providing therapeutic levels of stimulation intensity. In accordance with some embodiments, the neural fulcrum zone 852-1 can be located by identifying the zone in which the patient's response to stimulation produces either no heart rate change or a mildly decreased heart rate change (e.g., 5% decrease, or a target number of beats per minute). As the intensity of stimulation is further increased at the fixed first frequency, the patient enters an undesirable bradycardia zone 853-1. In these embodiments, the patient's heart rate response is used as an indicator of autonomic engagement. In other embodiments, other physiological responses may be used to indicate the zone of autonomic engagement at which the propagation of efferent and afferent action potentials are balanced.

FIG. 8B is a chart 860 illustrating a heart rate response in response to such a gradually increased intensity at two additional frequencies, in accordance with embodiments of the present invention. In this chart 860, the x-axis and y-axis represent the intensity level of the stimulation signal and the observed heart rate change, respectively, as in FIG. 8A, and the first set 810 of stimulation signals from FIG. 8A is also shown.

A second set 810 of stimulation signals is delivered at a second frequency lower than the first frequency (e.g., 5 Hz). Initially, as the intensity (e.g., output current amplitude) is increased, a tachycardia zone 851-2 is observed, during which period, the patient experiences a mild tachycardia. As the intensity continues to be increased for subsequent stimulation signals, the patient's heart rate response begins to decrease and eventually enters a bradycardia zone 853-2, in which a bradycardia response is observed in response to the stimulation signals. The low frequency of the stimulation signal in the second set 820 of stimulation signals limits the functional effects of nerve fiber recruitment and, as a result, the heart response remains relatively limited. Although this low frequency stimulation results in minimal side effects, the stimulation intensity is too low to result in effective recruitment of nerve fibers and engagement of the autonomic nervous system. As a result, a therapeutic level of stimulation is not delivered.

A third set of 830 of stimulation signals is delivered at a third frequency higher than the first and second frequencies (e.g., 20 Hz). As with the first set 810 and second set 820, at lower intensities, the patient first experiences a tachycardia zone 851-3. At this higher frequency, the level of increased heart rate is undesirable. As the intensity is further increased, the heart rate decreases, similar to the decrease at the first and second frequencies but at a much higher rate. The patient first enters the neural fulcrum zone 852-3 and then the undesirable bradycardia zone 853-3. Because the slope of the curve for the third set 830 is much steeper than the second set 820, the region in which the patient's heart rate response is between 0% and −5% (e.g., the neural fulcrum zone 852-3) is much narrower than the neural fulcrum zone 852-2 for the second set 820. Accordingly, when testing different operational parameter settings for a patient by increasing the output current amplitude by incremental steps, it can be more difficult to locate an amplitude in the neural fulcrum zone 852-3. It is likely that the patient will traverse from the tachycardia zone 851-3 to the undesirable bradycardia zone 853-3 in a single step. At that point, the clinician would need to reduce the amplitude by a smaller increment in order to produce the desired heart rate response for the neural fulcrum zone 852-3.

FIG. 8C is a chart 880 illustrating mean heart rate response surfaces in conscious, normal dogs during 14 second periods of right cervical vagus VNS stimulation ON-time. The heart rate responses shown in z-axis represent the percentage heart rate change from the baseline heart rate at various sets of VNS parameters, with the pulse width the pulse width set at 250 μsec, the pulse amplitude ranging from 0 mA to 3.5 mA (provided by the x-axis) and the pulse frequency ranging from 2 Hz to 20 Hz (provided by the y-axis). Curve 890 roughly represents the range of stimulation amplitude and frequency parameters at which a null response (i.e., 0% heart rate change from baseline) is produced. This null response curve 890 is characterized by the opposition of functional responses (e.g., tachycardia and bradycardia) arising from afferent and efferent activation.

FIG. 9 illustrates a method of operating an implantable medical device (IMD) comprising neurostimulator coupled to an electrode assembly. This method can be implemented using, for example, the VNS systems described above.

In step 901, the IMD is activated to deliver to the patient a plurality of stimulation signals at a first frequency (e.g., 2 Hz, as described above with respect to FIG. 9). Each of the plurality of stimulation signals is delivered having at least one operational parameter setting different than the other stimulation signals. For example, as described above, the output current amplitude is gradually increased while maintaining a fixed frequency. In other embodiments, different parameters may be adjusted to increase the intensity of stimulation at a fixed frequency.

In step 902, the patient's physiological response is monitored. In the example described above with respect to FIG. 8, the physiological response being observed is the patient's basal heart rate during stimulation at the various intensities at the first frequency. The physiological response may be measured using an implanted or external physiological sensor, such as, e.g., an implanted heart rate monitor 31, as well as other available physiological data, for instance, as derivable from an endocardial electrogram.

In step 903, the neural fulcrum zone for that first frequency is identified. In the example described above with respect to FIG. 8, the neural fulcrum zone corresponds to the range of stimulation parameter settings that result in a heart rate change of about 0% to about a decrease of 5%. In other embodiments, a different range of target heart rate changes or other physiological responses may be used to identify the neural fulcrum zone.

In accordance with some embodiments, stimulation at multiple frequencies may be delivered to the patient. In step 904, the IMD is activated to deliver to the patient a plurality of stimulation signals at a second frequency. In step 905, the patient's physiological response (e.g., basal heart rate) at the second frequency is observed. In step 906, the neural fulcrum zone for the second frequency is identified. Additional frequencies may be delivered and corresponding neural fulcrum zones may be identified for those frequencies.

As described in the various embodiments above, neural fulcrum zones may be identified for a patient. Different neural fulcrum zones may be identified using different stimulation signal characteristics. Based on the signal characteristics, the patient's physiological response to the stimulation may be mild with a low slope, as with, for example, the first set of stimulation signals 810 at a low frequency, or may be extreme with a large slope, as with, for example, the third set of stimulation signals 830 at a high frequency. Accordingly, it may be advantageous to identify a frequency at which the reaction is moderate, producing a moderate slope corresponding to a wide neural fulcrum zone in which therapeutically effective stimulation may be provided to the patient.

The observation of tachycardia in the tachycardia zone 851-2 and bradycardia in the bradycardia zone 853-2 indicates that the stimulation is engaging the autonomic nervous system, which suggests that a therapeutically effective intensity is being delivered. Typically, clinicians have assumed that stimulation must be delivered at intensity levels where a significant physiological response is detected. However, by selecting an operational parameter set in the neural fulcrum zone 852-2 that lies between the tachycardia 851-2 and the bradycardia zone 853-2, the autonomic nervous system may still be engaged without risking the dangers of either tachycardia or bradycardia. At certain low frequencies, the bradycardia zone may not be present, in which case the neural fulcrum zone 852-2 is located adjacent to the tachycardia zone. While providing stimulation in the neural fulcrum zone, the autonomic nervous system remains engaged, but the afferent and efferent activation are at sufficient balance that the heart rate response is minimized. Ongoing stimulation therapy may then be delivered to the patient at a fixed intensity within the neural fulcrum zone.

While the invention has been particularly shown and described as referenced to the embodiments thereof, those skilled in the art will understand that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope. For example, in various embodiments described above, the stimulation is applied to the vagus nerve. Alternatively, spinal cord stimulation (SCS) may be used in place of or in addition to vagus nerve stimulation for the above-described therapies. SCS may utilize stimulating electrodes implanted in the epidural space, an electrical pulse generator implanted in the lower abdominal area or gluteal region, and conducting wires coupling the stimulating electrodes to the generator. 

What is claimed is:
 1. A method of operating an implantable medical device (IMD) comprising neurostimulator coupled to an electrode assembly, said neurostimulator adapted to deliver a stimulation signal to a patient, said method comprising: activating the IMD to deliver to the patient a plurality of stimulation signals at a first frequency, each stimulation signal having at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals; monitoring a heart rate of the patient during said delivery of the plurality of stimulation signals, wherein tachycardia is detected during a first subset of the plurality of stimulation signals and bradycardia is detected during a second subset of the plurality of stimulation signals; and identifying a neural fulcrum zone corresponding to a third subset of the plurality of stimulation signals during which a transition heart rate response is detected, said transition heart rate response being between tachycardia and bradycardia.
 2. A method according to claim 1, further comprising: activating the IMD to chronically deliver stimulation signals in the identified neural fulcrum zone.
 3. A method according to claim 2, wherein said activating the IMD to chronically deliver stimulation signals in the identified neural fulcrum zone comprises activating the IMD to treat chronic cardiac dysfunction.
 4. A method according to claim 1, wherein: said at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals comprises amplitude.
 5. A method according to claim 1, wherein: said at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals comprises pulse width.
 6. A method according to claim 1, wherein: said at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals comprises duty cycle.
 7. A method according to claim 1, wherein: said at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals comprises frequency.
 8. A method according to claim 1, wherein: said activating the IMD to deliver to the patient the plurality of stimulation signals at the first frequency comprises activating the IMD to deliver to the patient the plurality of stimulation signals, each stimulation signal comprising a greater amplitude than the immediately previous stimulation signal.
 9. A method according to claim 1, wherein: the transition heart rate response comprises a heart rate change of between 0% and 5% reduced heart rate.
 10. A method according to claim 1, further comprising: activating the IMD to deliver to the patient a second plurality of stimulation signals at a second frequency different than the first frequency, each stimulation signal in the second plurality of stimulation signals having at least one parameter setting different than the parameter settings of the other stimulation signals in the second plurality of stimulation signals; monitoring a heart rate of the patient during said delivery of the second plurality of stimulation signals, wherein the tachycardia is detected during a first subset of the second plurality of stimulation signals and the bradycardia is detected during a second subset of the second plurality of stimulation signals; and identifying a second neural fulcrum zone corresponding to a third subset of the second plurality of stimulation signals during which the transition heart rate response is detected.
 11. A method according to claim 10, further comprising: activating the IMD to chronically deliver stimulation signals in the identified second neural fulcrum zone.
 12. A method according to claim 11, wherein said activating the IMD to chronically deliver stimulation signals in the identified second neural fulcrum zone comprises activating the IMD to treat chronic cardiac dysfunction.
 13. A method according to claim 1, further comprising: prior to activating the IMD to deliver to the patient the plurality of stimulation signals at the first frequency, completing a titration process in which stimulation intensity is gradually increased over a period of time to adapt the patient to the stimulation.
 14. A method according to claim 1, wherein: said IMD is adapted to deliver the stimulation signal to a vagus nerve of the patient.
 15. A method of identifying a set of parameter settings for an implantable medical device (IMD) for treating chronic cardiac dysfunction, comprising: activating the IMD to deliver to the patient a plurality of stimulation signals at a first frequency, each stimulation signal having at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals; monitoring a heart rate of the patient during said delivery of the plurality of stimulation signals, wherein tachycardia is detected during a first subset of the plurality of stimulation signals and bradycardia is detected during a second subset of the plurality of stimulation signals; and identifying a neural fulcrum zone corresponding to a third subset of the plurality of stimulation signals during which a transition heart rate response is detected, said transition heart rate response being between the tachycardia and the bradycardia.
 16. A neurostimulation system, comprising: an implantable medical device (IMD) comprising neurostimulator coupled to an electrode assembly, said neurostimulator adapted to deliver a stimulation signal to a patient; and a control system programmed to: monitor a heart rate of a patient implanted with the IMD during delivery to the patient a plurality of stimulation signals at a first frequency, each stimulation signal having at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals; identify a first subset of the plurality of stimulation signals during which tachycardia is detected; identify a second subset of the plurality of stimulation signals during which bradycardia is detected; and identify a neural fulcrum zone corresponding to a third subset of the plurality of stimulation signals during which a transition heart rate response is detected, said transition heart rate response between tachycardia and bradycardia.
 17. A system according to claim 16, wherein the control system is further programmed to: activate the IMD to chronically deliver stimulation signals in the identified neural fulcrum zone.
 18. A system according to claim 17, wherein the control system is programmed to activate the IMD to chronically deliver stimulation signals in the identified neural fulcrum zone by activating the IMD to treat chronic cardiac dysfunction.
 19. A system according to claim 16, wherein: said at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals comprises amplitude.
 20. A system according to claim 16, wherein: said at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals comprises pulse width.
 21. A system according to claim 16, wherein: said at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals comprises duty cycle.
 22. A system according to claim 16, wherein: said at least one parameter setting different than the parameter settings of the other stimulation signals in the plurality of stimulation signals comprises frequency.
 23. A system according to claim 16, wherein the control system is further programmed to: activate the IMD to deliver to the patient the plurality of stimulation signals, each stimulation signal comprising a greater amplitude than the immediately previous stimulation signal.
 24. A system according to claim 16, wherein: the transition heart rate response comprises a heart rate change of between 0% and 5% reduced heart rate.
 25. A system according to claim 16, wherein the control system is further programmed to: activate the IMD to deliver to the patient a second plurality of stimulation signals at a second frequency different than the first frequency, each stimulation signal in the second plurality of stimulation signals having at least one parameter setting different than the parameter settings of the other stimulation signals in the second plurality of stimulation signals; monitor a heart rate of the patient during said delivery of the second plurality of stimulation signals, wherein the tachycardia is detected during a first subset of the second plurality of stimulation signals and the bradycardia is detected during a second subset of the second plurality of stimulation signals; and identify a second neural fulcrum zone corresponding to a third subset of the second plurality of stimulation signals during which the transition heart rate response is detected.
 26. A system according to claim 25, wherein the control system is further programmed to: activate the IMD to chronically deliver stimulation signals in the identified second neural fulcrum zone.
 27. A system according to claim 26, wherein the control system is configured to activate the IMD to chronically deliver stimulation signals in the identified second neural fulcrum zone by activating the IMD to treat chronic cardiac dysfunction.
 28. A system according to claim 16, wherein the control system is further programmed to: prior to activating the IMD to deliver to the patient the plurality of stimulation signals at the first frequency, complete a titration process in which stimulation intensity is gradually increased over a period of time to adapt the patient to the stimulation.
 29. A system according to claim 16, wherein: said IMD is adapted to deliver the stimulation signal to a vagus nerve of the patient. 